Pertussis Vaccine Helps Reduce Transmission of Disease, According
to Emory and Senegal Researchers

ATLANTA — Vaccination for pertussis (whooping cough) is highly effective
in reducing transmission of the disease from person to person, even
in "breakthrough" cases in which a vaccinated individual still develops
pertussis, according to research conducted by scientists at Emory University
and in Dakar, Senegal. The finding should help resolve a long-standing
debate about the effectiveness of the vaccine in reducing contagiousness
of the disease in addition to conferring immunity.

The research was conducted
by Marie-Pierre Préziosi, MD, PhD, former visiting faculty member at
Emory University’s Rollins School of Public Health and the Institut
de Recherche pour le Développement in Dakar, Senegal, and M. Elizabeth
Halloran, MD, DSc, professor of biostatistics at the Rollins School
of Public Health. The article was in the online edition of Vaccine January
14 and will be published in the printed journal on May 16.

The incidence of pertussis
is on the rise in rise in infants, adolescents and adults in the U.S.
and other developed countries, and millions of cases occur annually
in the developing world. To clarify and promote an effective vaccination
policy, scientists have needed a better understanding of how pertussis
is transmitted and how that is affected by vaccination.

Earlier studies had indicated
that vaccination might not alter the number of circulating Bordetella
pertussis bacteria and thus transmission of the disease, because the
period between epidemics did not appear to vary with the level of vaccination.
More recent studies, however, have shown that widespread vaccination
is, in fact, linked to longer periods between epidemics. Confirming
studies have shown that infants too young to be directly protected by
vaccination have decreased levels of pertussis disease in populations
with broader vaccination coverage and that incidence of pertussis in
parents and younger siblings of vaccinated children is lower than in
parents and siblings of unvaccinated children.

To measure the effectiveness
of pertussis vaccination in reducing transmission from vaccinated compared
with unvaccinated individuals, Drs. Préziosi and Halloran analyzed data
from a followup study within a rural community of 30,000 residents in
30 villages in Niakhar, Senegal, where active surveillance of pertussis
has been conducted since 1983. The investigators focused on the calendar
year 1993 (an epidemic year). In particular, they considered a group
of 3,021 children under age 15 that included 340 primary cases (index
cases) in households and 2006 contacts with no history of pertussis.
Of these contacts, 41% became suspected secondary cases.

The primary interest in the
analysis was to estimate the reduction in transmission from vaccinated
compared with unvaccinated primary cases. Vaccine efficacy for infectiousness
(VEi) was defined as the relative reduction in the secondary attack
rate (SAR) in contacts exposed to vaccinated cases compared to contacts
exposed to unvaccinated cases. Using the main case definition, the VEi
was a dramatic 85% with a 28-day cut-off and 67% with no cut-off period
for the development of secondary cases.

Total vaccine efficacy (VEt)
was defined as the relative reduction in the SAR when both the infectious
case and the contact were vaccinated compared to if both were unvaccinated.
The VEt was measured at 89% with a 28-day cut-off and 77% with no cut-off
for the development of secondary cases.

"This work includes important
and timely findings on the prevention of pertussis and provides a unique
insight into an effect of vaccination not traditionally estimated,"
said Dr. Préziosi. "The focus on vaccine efficacy for infectiousness
presents a dramatically new way to look at the effects of vaccination
from a true public health perspective, and clearly shows that vaccination
makes breakthrough cases less contagious."

"We hope this new evidence
­­ that pertussis vaccination not only provides immunity to those vaccinated,
but also a substantial indirect benefit in reducing transmission ­­
will give individuals and public health programs additional arguments
for vaccination," said Dr. Halloran.

This analysis was supported
by the National Institute of Allergy and Infectious Disease and the
French Ministry of Foreign Affairs, the Singer-Polignanc Foundation,
and Aventis Pasteur. Data collection was financed by Institut de Recherche
pour le Développement and Aventis Pasteur.